How to play a video strip backward in blender 2.8?
Check out your FPS readout in the Preview pane and the cache indicator in the Sequence view. 

If your FPS is flashing red numbers that are lower rates than the source video, your machine's struggling to keep up. 



Play in reverse Shift-Ctrl-Spacebar usually lags for me until the clip is mostly cached. You'll see the clip cache build as an orange bar below the tracks in Sequence. 



Look into cache allocation via Blender's Preferences ... System ... Sequencer Cache Limit and into Proxies. These and other tips will help you squeeze more performance from your machine.